The present invention is concerned with an improved method and product for the removal of sulfur-containing compounds having disagreeable odors, from gas streams containing oxygen, most often, air. The improvement comprises use of a suitable activated carbon impregnated with NaOH, lead acetate (hereinafter PbOAc), and moisture.
Copending application Ser. No. 752,498, the subject matter of which is incorporated herein by reference, discloses improved removal of malodorous compounds using activated carbon impregnated with NaOH and moisture.
The malodorous sulfur-containing compounds whose removal is enhanced by use of the method and product of the present invention include, principally, hydrogen sulfide, and mercaptans, especially methyl mercaptan.
Malodorous sulfur-containing compounds occur in a number of environments, where they are primarily responsible for the presence of disagreeable odors. Such environments include petroleum storage areas, sewage treatment facilities, and pulp and paper production sites, among others. These odor-causing compounds may be bacterial breakdown products of higher organic compounds.
Hydrogen sulfide, H.sub.2 S, is a colorless, denser than air, gas with a characteristic odor of rotten eggs. The gas is produced in coal pits, gas wells, sulfur springs, and from decaying organic matter containing sulfur. Control of emissions of this gas, particularly from municipal sewage treatment plants, has long been considered desirable. In fact, in addition to its unpleasant odor, hydrogen sulfide is not only flammable, but with respect to its human toxicity, it is regarded as an extremely hazardous substance. Consequently, ways of controlling hydrogen sulfide emissions have long been sought in a number of areas, including, among others, crude oil storage, petrochemical refining, and papermaking.
Mercaptans, also referred to as thio alcohols or thiols, may be represented by the formula R-S-H, where R represents an alkyl group of from one to eight carbon atoms. The obnoxious odor of mercaptans decreases with increasing molecular weight, and is not present where R is alkyl of nine or more carbon atoms. While only methyl and ethyl mercaptan of the said mercaptans are in the form of a gas at normally encountered ambient temperatures, the mercaptans are volatile and even extremely small concentrations are detectable by humans. Consequently, mercaptans of up to eight carbon atoms are included within the scope of the present invention. Nevertheless, the present invention is particularly useful for removal of methyl mercaptan.
Activated carbon will physically adsorb considerable quantities of hydrogen sulfide. See, for example, U.S. Pat. No. 2,967,587. See also French Pat. No. 1,443,080, which describes adsorption of hydrogen sulfide directly by activated carbon, which is then regenerated by hot inert gas or superheated steam. However, it has long been recognized that much better removal can be accomplished by employing the carbon to, in effect, catalyze the oxidation of hydrogen sulfide to sulfur, based on the ability of carbon to oxidize hydrogen sulfide to elemental sulfur in the presence of oxygen. For example, a conventional process employs small amounts of ammonia added to the influent gas stream containing hydrogen sulfide and oxygen to further catalyze the reaction, and capacities as high as 100 percent by weight of the carbon have been reported.
Other catalytic agents to be impregnated on activated carbon for the removal of hydrogen sulfide have been suggested. See, for example, French Patent No. 1,388,453 which describes activated carbon granules impregnated with 1% iodine (I.sub.2) for this use. South African Patent No. 70/4611 discloses the use of silicate-impregnated activated carbon. Swinarski et al., Chem. Stosowana, Ser. A 9(3), 287-94 (1965), (Chemical Abstracts, Vol. 64, 1379c), describe the use of activated carbon treated with potassium salts, including KOH, for hydrogen sulfide adsorption.
A problem faced in the prior art in using activated carbon for hydrogen sulfide removal has been the reduction in net adsorption capacity of the activated carbon with increasing number of adsorption cycles. That is, the activated carbon experiences an increase in the amount of residual compositions, possibly sulfur, which, in turn, results in a continuing reduction in the total adsorption capacity of the activated carbon through successive adsorption cycles. South African Patent No. 70/4611, referred to above, teaches overcoming this problem with the use of silicate-impregnated activated carbon, but also teaches that extraction with alkaline solutions is ineffective to remove such residual adsorbate. Boki, in Shikoku Igaku Zasshi, 30 (3), 121-8 (1974) (Chemical Abstracts, Vol. 81, 126300p), discloses that the absorption capacity of activated carbons used for adsorption removal of hydrogen sulfide gas can be recovered to nearly original levels by treatment with 1% NaOH. However, it is clear that these are simply attempts in the prior art to overcome the problem of decreasing net adsorption. Thus, the prior art has failed to appreciate the discovery of the present invention, that proper treatment of activated carbon with NaOH, PbOAc and moisture can result in an activated carbon whose capacity for malodorous sulfur-containing compound removal is increased several fold, not merely restored to unused (virgin) activated carbon capacity. In accordance with the present invention, the character of the adsorption by the activated carbon is apparently changed from predominantly physical adsorption to predominantly chemical reaction followed by physical adsorption.
The overall reaction in which hydrogen sulfide is oxidized to elemental sulfur in the presence of activated carbon may be represented by the following equation: EQU 2H.sub.2 S + O.sub.2 -- 2H.sub.2 O + 2S (1)
However, it has been demonstrated that two other reactions can occur: EQU 2H.sub.2 S + 3O.sub.2 -- 2SO.sub.2 + 2H.sub.2 O (2)
and EQU 2SO.sub.2 + 4H.sub.2 S -- 6S + 4H.sub.2 O (3)
it has been demonstrated that the reaction (2) above is accelerated by the presence of moisture on the activated carbon. See Swinarski, A., and Siedlewski, J. Roczniki Chemii, 35, pp. 999-1008 (1961) and U.S. Pat. Nos. 3,598,521 and 3,416,293. It is also known that preoxidation of the carbon surface increases total hydrogen sulfide removal capacity, but at the same time also increases the proportion of sulfur oxides formed. Thus, during hydrogen sulfide removal by activated carbon, a number of potential reaction products are possible, although the primary reaction product is elemental sulfur.
Various methods are known in the art for removing malodorous sulfur compounds from gas streams using impregnated activated carbons. For example, see Japanese Patent Application No. 39-23720 where carbonate of hydroxide of potassium or sodium is used to impregnate the activated carbon. U.S. Pat. No. 3,391,988 discloses a method for removal of mercaptans from exhaust gas by contact with an adsorbent impregnated with a liquid mixture of an alkaline material.
The present invention is also indirectly concerned with the use of NaOH to regenerate activated carbon which has become loaded with, i.e., reached its efficient removal capacity for malodorous sulfur-containing compounds. The regeneration with NaOH restores the major part of the original adsorptive capacity of the spent activated carbon. However, the present invention employs such regeneration as a convenient method of removing adsorbate while at the same time providing a ready means of impregnation of the activated carbon with NaOH for improved adsorptive capacity for malodorous compounds, as described hereinafter. Thus, other regeneration techniques, known in the art, might be employed, followed by NaOH, PbOAc and moisture impregnation. Such conventional regeneration techniques, useful in restoring virgin capacity prior to impregnation with NaOH, PbOAc and moisture, include thermal treatment and wet air oxidation, among others.
It is well known that activated carbons used for removal of hydrogen sulfide can be regenerated for re-use by removing the adsorbed sulfur compounds, a large portion of which will be elemental sulfur when oxidizing conditions exist during the absorption. The compounds can be removed by extracting them with a suitable organic solvent. Such materials as ammonium sulfide, carbon disulfide, xylene and toluene have proven effective regenerating media. Regeneration using ammonium sulfide as the solvent has been common. Sulfur is recovered from the solvent by distilling off the ammonium polysulfide or by steaming the solvent. Regeneration has also been accomplished using hot inert gas, superheated steam and natural gas under high pressure.
Regeneration methods heretofore employed in the art for restoring activated carbons used to remove hydrogen sulfide suffer from a number of disadvantages. The use of organic solvents is undesirable from the standpoint of environmental pollution as well as of personnel safety, and will usually entail expensive recovery systems. Regeneration by hot gases or steam requires the expenditure of considerable amounts of energy, a clear disadvantage. A more serious detriment, perhaps, than those just discussed, is that detriment inherent in most prior art activated carbons used for hydrogen sulfide removal and processes for their regeneration, which is the decrease in net adsorption capacity experienced through successive adsorption cycles. The disadvantages of these prior art activated carbons and regeneration processes as been overcome in the treatment method and resultant activated carbon of the present invention.